A bicycle dynamo that generates electricity contactlessly is disclosed in United States Patent Application Publication No. 2014/0132155 (“the '155 Publication”). In a bicycle dynamo of the '155 Publication, an outer peripheral surface of annular permanent magnet that rotates about a rotation axis extending in a direction orthogonal to a rotation axis of a bicycle wheel is spaced apart from a side surface of the wheel that is continues with an outer peripheral surface of the wheel.
In the permanent magnet, magnetic poles are aligned in the circumferential direction and the magnetization directions of adjacent magnetic poles are opposite to one another. For example, when the wheel rotates with the N pole of the permanent magnet facing the side surface of the wheel, eddy currents are generated on the side surface of the wheel in such a direction as to hinder the change of the magnetic flux from the permanent magnet. A repulsive force and attractive force between the magnetic flux generated by the eddy currents and the magnetic flux from the permanent magnet cause the permanent magnet to rotate in the rotation direction of the wheel.
Thus, when a coil is wound around the permanent magnet and the magnetic flux from the permanent magnet is linked with the coil, it is possible to obtain an induced electric power from the coil.
However, the bicycle dynamo disclosed in the '155 Publication has the following drawbacks.
1. Due to a limited area of the permanent magnet opposed to a side surface of the wheel, the amount of magnetic coupling between the wheel and the permanent magnet cannot be increased. Accordingly eddy currents generated in the wheel cannot be increased, and a rotational force of the permanent magnet becomes relatively weak.
2. In the '155 Publication, a single phase coil is wound around the permanent magnet. However, with the single phase coil, the magnetic flux from a part of the permanent magnet where the coil is not wound cannot be effectively utilized and the amount of the linkage magnetic flux cannot be increased. Moreover, in the case where the polarities of portions of the permanent magnet where the coil is wound around are symmetrical with respect to the rotation axis, the net amount of the magnetic fluxes linked with the coil is always canceled out so that it is not possible to generate electric power.
3. Because the magnetic flux from the permanent magnet propagates in the air, it receives a large magnetic resistance and therefore a high magnetic efficiency cannot be obtained.
4. Because a yoke is not utilized, leakage of magnetic flux is likely to occur. Further when an electrically conductive material is placed therearound, a magnetic path is changed by the material and it may affect the power generation amount.
In view of the above-described drawbacks, it is an object of the invention to provide a non-contact power generator with high magnetic efficiency and less leakage of magnetic flux.